Exactly, nvidia are fully aware of this. AMD don't really care because they have their fingers in every single pie there is so mining crash/second hand desktop gpu market won't bother them especially since their market share in this area is dropping even further now....To a point yes however Nvidia is worried that they will end up with the same situation they had with the 2 series cards. At some point this boom will crash and all these 3 series cards will start flooding the market just as Nvidia release the 4 series. People then jump on the second hand cards and less people will end up buying the 4 series.
I know it sounds bad..... but I personally can't wait to see greedy etailers etc. suffer come the mining crash, it's not a question as to if but when.